pf unconditionally disables TCP checksum offloading

[J David]

Hello,

While testing 9.2, we discovered that merely having pf enabled (no
rules of any kind) was sufficient to completely, unconditionally
disables hardware checksum offloading.

If pf is disabled ("pfctl -d") then checksum offloading works fine.
If pf is merely enabled with no rules ("pfctl -e -F all"), checksum
offloading no longer works.

The culprit appears to be this code in pf_check_out:

        if ((*m)->m_pkthdr.csum_flags & CSUM_DELAY_DATA) {
                in_delayed_cksum(*m);
                (*m)->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA;
        }

CSUM_DELAY_DATA is defined as (CSUM_TCP | CSUM_UDP) in sys/mbuf.h, so
this effectively clears (*m)->m_pkthdr.csum_flags and never puts it
back.

Is this behavior intentional?

Thanks!